The present invention relates generally to devices for burn-in and testing of integrated circuit chips (IC) or other devices and more specifically to techniques for cooling the ICs on the burn-in boards that are used to ensure that newly-manufactured ICs are suitable for use. Still more particularly, the present invention comprises a socket that provides an improved system for cooling the device under test and is capable of accommodating a variety of discrepancies in the presentation of the device under test.
It is well-known in the art of electronic device manufacturing to test, and/or xe2x80x9cburn-in,xe2x80x9d various electronic sub-components before assembling them into a larger device. For example, computer chips are frequently individually connected in a burn-in system for the purpose of ensuring that all of the desired electronic circuits in each chip are operational. The burn-in process accelerates aging of the chips and thus allows defective chips to be identified and discarded early in the manufacturing process. This is desirable because it allows the manufacturer to avoid the expense that would otherwise be wasted by constructing a larger, more expensive device containing the defective chip. In addition to burn-in, computer chips and other integrated circuits may be subjected to various other testing operations. The term xe2x80x9ctestingxe2x80x9d as used herein is intended to encompass and include burn-in operations.
In a burn-in operation, each chip, integrated circuit (IC), or other electronic component, each of which is hereinafter referred to as a xe2x80x9cdevice under testxe2x80x9d or xe2x80x9cDUT,xe2x80x9d is connected to several electronic leads. These leads typically take the form of an array of small solder buttons that are positioned to correspond to electronic leads on the under-surface of the DUT. The DUT is placed on the arrayed leads so that an electrical connection is made at each desired point.
During a burn-in or test operation, heat is generated by the passage of current via the leads through the various circuits on the DUT. Heretofore, ICs were less powerful and, correspondingly, the amount of power consumed during burn-in of a computer chip was relatively small. For this reason, the amount of heat generated was such that burn-in devices could be air-cooled in most cases. With the advent of newer, more powerful chips, the amount of heat generated during burn-in has multiplied ten-fold, from about 3-10 watts, to 30-100 watts or more.
In addition, the increasing cost of chip packaging has motivated manufacturers to advance the burn-in step so that it is carried out before, rather than after, final packaging. This allows manufacturers to save the cost of packaging a defective chip, but means that the burn-in operation must be carried out on partially packaged ICs, where the silicon die itself may be exposed. Partially packaged ICs are less robust and more susceptible to damage than fully packaged chips. Thus, the burn-in operation cannot subject the DUTs to excessive or uneven forces.
Because the burn-in must be carried out at a controlled temperature, and because the chips cannot be exposed to temperature extremes, it is imperative that the significant heat generated during burn-in be removed. Air cooling does not provide sufficient cooling without a very large heat sink. Liquid cooling, using an electrically insulating fluid has been tried, but has proven nonviable for very high power DUTs. At the same time, burning-in or testing a partially packaged chip raises new considerations over burning-in or testing a fully packaged chip. For example, partially packaged chips are not typically adapted to readily dump heat at the rate required.
It is known that high-power transistors generate comparable amounts of heat during burn-in operations. However, the configuration of transistors and conventional transistor packages is such that cooling systems that are designed for transistor burn-in devices cannot readily be adapted to cool IC burn-in devices. In addition, transistors are typically sealed within durable metal or plastic packages, so that the handling concerns that arise in the context of burning in chips do not arise in transistor burn-in devices. Furthermore, as compared to the volume of high power transistors that require testing, the volume of computer chips that must be tested is so many times greater that cost factors that are not significant in the context of transistor testing become prohibitive when contemplated in the context of chip testing.
In addition to the problems associated with providing sufficient cooling capacity to a given burn-in device and providing a heat transfer surface does not limit that capacity, problems arise from the fact that the amount of heat generated during burn-in or testing varies significantly from DUT to DUT. It has been found that in some instances, the amount of heat generated varies by as much as two orders of magnitude. This variance make it difficult to simultaneously burn-in several devices, as a cooling system that adequately cools the DUTs that generate greater amounts of heat will over-cool the DUTs that generate less heat, causing their temperatures to fall below the desired burn-in temperature range. Conversely, a cooling system that properly cools the DUTs that generate lesser amounts of heat will under-cool the DUTs that generate more heat, causing their temperatures to rise above the desired burn-in temperature range.
Furthermore, in addition to the operational variation between DUTs, it has been found that the physical configurations of the DUTs vary significantly. Specifically, each dimension or parameter relating to the DUT, including die thickness, device thickness, squareness, and thermal expansion, is typically specified to be within a prescribed range or tolerance. The overall configuration of a given DUT reflects the cumulative deviation of all the parameters from their target values. For this reason, even if all parameters on a particular DUT are within their prescribed ranges or tolerances, if the deviations do not offset each other there exists the possibility that particular DUT will present a heat transfer surface that is extremely and unacceptably misaligned.
It is desired to provide a DUT burn-in device that is capable of simultaneously removing at least 30-100 watts of heat from each of several chips, while maintaining the temperature of each DUT within a narrow desired range. In order to accomplish this heat transfer, good thermal contact must be provided between each DUT and a cooling system. In addition, it is often necessary to apply a certain compressive force to each DUT, in order to achieve proper electrical contact between the leads and the DUT.
Hence, it is desired to provide a burn-in device that can provide the necessary good thermal contact while at the same time accommodating differently sized and shaped DUTs and providing the necessary compressive force to each. The preferred device should be capable of maintaining the DUTs within the prescribed temperature even though the DUTs produce amounts of heat that may vary by more than an order of magnitude and even though some DUTs may generate as little as 3 watts of heat. The preferred device should also be readily incorporated into a system capable of simultaneously processing multiple DUTs. These objectives require that the device be capable of compensating for variance in heat generation between DUTs that are being burned in simultaneously. The preferred device should be able to handle unpackaged chips without damaging them before, during or after the burn-in process. It is further desired to provide a burn-in device that is commercially viable in terms of cost, labor and reliability.
The present invention comprises a burn-in device that is capable of simultaneously removing at least 30-100 watts of heat from each of several DUTs, while compensating for variations in DUT configuration and presentation. The present device also compensates for variation in heat generation between DUTs and maintains the temperature of each DUT within a to narrow desired range. The present invention is readily incorporated into a system capable of simultaneously burning-in multiple DUTs. The preferred device causes a minimum of damage to the DUTs and is commercially viable in terms of cost, labor and reliability.
The present invention comprises a novel socket for receiving and contacting an individual chip during burn-in, and to a system for supporting and cooling several of the sockets. The socket includes a cooling system that is capable of removing at least 3 to 10 times as much heat from a DUT as previous systems. A preferred embodiment includes a highly thermally conductive, mechanically biased, resiliently mounted heat spreader and at least one highly thermally conductive heat sink member held in good thermal contact with the integrated circuit or device-under-test (DUT). The interface between the adjustable resiliently mounted heat spreader and the cooling system is also designed to be highly thermally conductive and to allow good heat transfer despite variations in the position of the resiliently mounted heat spreader.
The present invention further includes apparatus and technique for achieving good thermal contact with the DUT. A preferred embodiment provides a conformal interface that conforms to any unevenness in the upper surface of the DUT. In a first embodiment, this thermal contact is obtained via an elastomeric heat pad and a heat spreader that together form the socket lid. The elastomeric heat pad is preferably covered by a thin metal film. In another embodiment, the conformal interface comprises a low melting point metal contained within a skin formed from a much higher melting point metal. In a less preferred embodiment, the interface comprises an ultra-smooth, highly polished metal surface.
A preferred embodiment of the present invention further includes a temperature sensor for monitoring and providing data on the temperature of the cooling system in the vicinity of the DUT and a heat source for applying a controlled amount of heat to the DUT in response to the output of the temperature sensor. The temperature sensor is preferably embedded in the heat spreader near the interface with the DUT. The heat source is preferably also embedded in the heat spreader and is preferably controlled by a controller in response to the signal generated by the temperature sensor.
A preferred embodiment of the present cooling system also includes a liquid-vapor cooling unit (LVU) in thermal contact with the heat sink and socket. The liquid-vapor cooling system preferably includes multiple liquid-vapor ducts controlled by a single controller, resulting in significant cost and operational savings over the prior art. In another embodiment, the liquid-vapor cooling system is replaced by a circulating liquid system, known as a liquid cooling unit (LCU). The LCU allows for burn-in temperatures of less than 60xc2x0 C.
According to the present invention, a separate burn-in socket receives each DUT. Each socket is preferably constructed such that the biasing force that allows good thermal contact between the heat sink and the DUT is controlled and distributed across the DUT, so as to avoid mechanical damage to the DUT. The preferred socket also provides means for applying sufficient contact force between the socket base and the DUT to allow for good electrical contact, while at the same time limiting the application of compressive force to the DUT so as to avoid damaging the DUT.